I'm seeing so many experts coming on TV and info-shows dispensing explanations -(on the Fukishima nuclear crisis)- that are inaccurate, I'm beginning to wonder exactly how much these people have researched their own field. I'm no nuclear expert but I can certainly spot a lot of the inaccuracies in the information -(read OPINIONS)- being dispensed. Some of the "dis-information" as well though, is a result of knowledgeable people saying LESS than than they suspect ... with their own reputations in mind and an honest concern for avoiding the creation of unnecessary panic. I don't fault the latter category of professionals for being cautious but the end result is that all of it STILL leaves most people in the dark as to the LIKELY outcome of the Fukishima reactor explosions.
In the interest of providing some basic understanding, I'm compiling the following essay on the mechanisms of nuclear reaction which will -hopefully- give the average person a better picture of what goes on in nuclear reactors.
First of all, lets examine the term "nuclear" to see what it means.
All matter on earth and in our universe is made up of tiny building blocks called atoms. This is where the expression "atomic" comes from. All atoms are made up of 2 basic components; a nucleus "ball" with electrons spinning around the nucleus in "shells" or orbits. (Think of the earth being a nucleus and satellites flying around the earth being electrons. This is how atoms are constructed. Note too
that all of this is still theoretical; no atom has ever actually been seen with the human eye. There is always the chance that the entire nuclear theory might be "off" ... but ... the theory has been tested and found to fit for the development of nuclear power and nuclear energy).
Let's look at the simplest atom in existence. It has a nucleus of one proton and one electron flying around it in the nearest shell (orbit). This is a hydrogen atom. The electron has a negative charge and the proton has a positive charge. The two charges attract each other and would unite to cancel each others' charges ... but the orbital spin of the electron creates a "centrifugal" pull outward ... which keeps the electron at a specific distance from the nucleus at all times ... just like an orbiting shuttle or satellite is kept in space above the earth. (Simplistic explanation).
Atoms need to have their outer shell of electrons stabilized by having an even number of electrons. The reason for this is beyond the scope of my explanation, so let's just accept that as a "fact."
Since hydrogen has only one electron in its ONLY shell (therefore ITS outer shell) ... it combines with another hydrogen atom and then the two atoms share their electrons. This then, creates a molecule
of hydrogen denoted by H2.
If we look at another gas -oxygen- it TOO needs to combine with "something" to create a stable non-active outer shell ... but it has more shells and a more complex nucleus. Here's a pictorial rendering of an oxygen molecule made up of two oxygen atoms, sharing outer shell electrons...
And then ... here's a pictorial of the water molecule ... where 2 hydrogen atoms and one oxygen molecule all combine through the sharing of their outer shell electrons...
That covers the (elementary) construction of atoms and their combination(s) into molecules by sharing shell electrons.
Now let's look at the nucleus or nuclei of certain atoms. Like satellites circling the earth in orbits, the satellites are very tiny in relation to the earth ... the same is true of atomic electrons being very tiny in relation to their nucleus.
The hydrogen atom has only a single proton as a nucleus. (There are variations on this; the heavy hydrogen atom has a neutron in the nucleus as well ... making it about twice as "heavy" as a standard hydrogen atom).
Hydrogen is the lightest element in the world, having only one proton in its nucleus. Electrons are thought to have no mass or negligible mass but protons DO have mass. Hydrogen has an atomic weight of one -one proton.
The next heavier element is helium. Helium has a 2 electrons in its shell and a set of 2 protons and 2 neutrons in its nucleus ... being about 4 times as heavy as hydrogen. (Remember helium because its nucleus is the cause or source of "alpha radiation".)
And finally, let's take a look at a uranium atom ... about the HEAVIEST atom there is!
Add up the number of protons and neutrons and you get the particular number of any "isotope" of uranium. Since there are numerous possibilities, these variations are called "isotopes" of that element. Uranium 238 -the most abundant isotope to be found in nature- has 92 electrons, 92 protons and 146 protons. U235, on the other hand -the most active ingredient in a nuclear reactor (and very scarce, making up a bit less than 1% of all natural uranium)- has 92 electrons, 92 protons and 143 neutrons.
NEUTRONS are of greatest interest in nuclear reactions because they are the cause of primary nuclear reaction to split atoms and create heat energy.
U235 (thankfully) is very diluted in natural uranium deposits in the world -(.7%). U235 nuclei are "unstable," meaning that they don't 'feel comfortable' with having an odd number of neutrons in their bellies and for the most part, they put up with the discomfort but OCCASIONALLY they split because of this imbalance and when they do, the entire atom splits and turns into 2 or more other elements and throws out 3 neutrons at very high speed.
Because U235 atoms only split up (fission) spontaneously occasionally
... there's very little of it going on in nature but ... it does happen on a continuing basis. Since there are trillions+ atoms of U235, the "occasional" means that it still happens regularly enough to measure with Geiger counters. AND ... over years and years and years, the AMOUNT of U235 gets steadily smaller. The depleting rate is calculated as "half life". Over X amount of years, half of the original amount will have destroyed itself by fissioning and eventually, ultimately, there will be none left.
The half life for U235 is 703,800,000 years. That means, in that much time, half will be gone. In another 703 million years, half of what was left, will also be gone ... and so on. At this point in time, it's pretty scarce with most of it having already spontaneously fissioned.
Now here's the curious -but key thing- about what happens when the U235 atom splits and expels its 3 neutrons at high speed ...
If these high speed neutrons hit the nucleus of other U235 atoms, they'll go straight through them like a high velocity steel-jacketed rifle bullet. They WON'T affect the nucleus at all! But if the high speed neutrons are slowed down
and then strike the nucleus of another (or other) U235 atom(s) ... they lodge inside the nucleus and make it very sick. The struck nucleus may split immediately or wait a short time and then split too ... but when it splits, it repeats the action of the first atom whose neutrons flew out.
This fly-out and capture and split sequence can be closely controlled with a moderator
-which SLOWS the neutrons- and .... control rods, which simply stop the neutrons ... to sustain a controlled chain reaction
that keeps going on and on. This, of course, means a very high amount of heat will be generated and this heat is taken away by water flowing through the fuel rods, acting as moderator and coolant, all at once. It's a "beautiful friendship" as Humphrey Bogart might say.
To sum up this process, it's something like one guy coming out with something original and then telling another person or persons about it ... but he talks so fast, it simply "goes in one ear and out the other." His idea goes absolutely nowhere except for himself. Then a moderator comes along and slows it down so that what he is saying registers
with the listeners! They in turn, then tell others about it very fast but a moderator slows their explanations down too .... and you get this chain reaction of information BUILDING and continuing outward. Only about 3% of the audience ever understand it regardless -so the majority will contribute nothing to propagation- but the 3% will be sufficient to keep it going and going to the ends of the earth, so long as a moderator is always in place to slow down the explanations.
But there's ANOTHER curious thing which happens as well ...
The fast-flying neutrons will actually penetrate the nuclei of U238 atoms "occasionally" as well! The U238 nuclei don't split but instead ... SWALLOW the neutron and then become fissionable too. (A U238 atom which absorbs a fast neutron becomes or turns into ... plutonium 239). After that, a fast-flying neutron entering into the the plutonium nucleus will cause it to fission as well and release a very large amount of energy from fissioning too.
I should point out that when atoms fission (split into other simpler elements) ... the WEIGHT of the new products is slightly less than the weight of the parent atom. This loss of weight is turned into energy ... bearing out Einstein's equation of E=mc2 .... of matter turning into energy.
So that's the end of this segment ... explaining how a nuclear reactor uses Uranium to generate a nuclear reaction. The key "trigger" in a nuclear reactor is the moderator slowing down fast-flying neutrons from splitting U235 atoms so that they can be captured by other U235 atoms to make them split, spew out more fast neutrons which are re-slowed to keep the chain reaction going continuously. A byproduct of the process is the "occasional" capture of fast-flying neutrons by U283 atoms which turns U238 into Pu 239 ... which is then fissioned by more fast-flying neutrons to generate additional heat.